docs: def df_pdb_ranking(self):
          """DataFrame: Get a dataframe of UniProt -> best structure in PDB results"""
          df = pd.DataFrame()
          for g in self.genes_with_experimental_structures:
              protein_df = g.protein.df_pdb_ranking.copy().reset_index()
              if not protein_df.empty:
                  protein_df['gene'] = g.id
                  df = df.append(protein_df)
          if df.empty:
              log.warning('Empty dataframe')
              return df
          else:
              return ssbio.utils.clean_df(df.set_index('gene'))